An arrangement including a transmitter/receiver component fixedly mounted to a chassis and a reflector component fixedly mounted on an axle is disclosed, for example, in German patent publication 3,620,957.
Within the bellows of an air spring, distances of approximately 0 to 600 mm in length are to be measured at temperatures between -400.degree. C. and +120.degree. C. to determine the distance between the chassis of a vehicle and the axle. An advantage of utilizing ultrasound within an air spring is that no turbulence is imparted to the sound waves because of the wind generated during movement of the vehicle.
German patent publication 3,620,957 discloses an air spring system which includes a measuring device for determining the spacing between a first part of the spring fixedly mounted to the chassis and a second part of the spring fixedly mounted to the axle. The measuring device comprises an ultrasonic transducer for transmitting a series of pulses in the axial direction of the spring. Furthermore, means are provided to receive these axially directed pulses after the pulses have traveled at least once through the distance of the interior of the spring. Preferably, the second end element is a reflector and the receiver is disposed in the region of the transmitter.
The air springs, which are equipped with such a transmitter/receiver unit, were found to be unsuitable in practice. It turned out to be a disadvantage that the transmitter/receiver unit is disposed centrally on the longitudinal axis on the upper part of the air spring. In order to provide the necessary structural space for accommodating this part and its feed line, it is necessary to bore the longitudinal support of the vehicle disposed centrally above the air spring or to provide a corresponding cutout therein. This requires additional manufacturing steps and mounting is difficult. If one would arrange the transmitter/receiver unit and reflector off center, then there would be no reproducible path distance to the reflector because the deflection or movement of an air spring does not take place exclusively in the axial direction.
Also, for an off-center arrangement, the transmitted beam would not be reflected back again to its starting point because air springs move (as a consequence of the elastic axial attachment) during spring action along a spatial curve which cannot be determined beforehand with precision, that is, angles and lateral displacements are not clearly known. The wobbling movements would operate on the reflections.
If one would transmit to an off-axis surface utilized as a reflector, then a non-reproducible change of the geometric relationships would occur because of angle changes between the part fixed to the chassis and the part of the air spring fixed to the axle. This would be the case especially for a one-sided loading of the vehicle and during travel of the vehicle in a curve. Providing a center axial distance between the part fixed to the chassis and the part fixed to the axle would in no event be possible. If one would attempt to overcome this unsuitable condition by installing a pair of transmitter/receiver-reflector units, then the complexity of the apparatus would double.